In recent years, regarding displays of liquid crystal display devices and the like, optical sheets that are provided with fine uneven shapes on the surfaces and that realize predetermined functions by refracting light with the uneven structures have been indispensable and the performance, e.g., a high refractive index or a shape maintaining force, is required. Examples of such optical sheets include a Fresnel lens sheet and a lenticular sheet used as projection screens, e.g., a projection television, a prism sheet and a microlens sheet used for a backlight of a liquid crystal display device and the like, and a moth-eye film used as an antireflection film of a flat-screen television in recent years.
For example, a resin material for forming a prism sheet (optical sheet) used for a backlight of a liquid crystal display device or the like is required to have, in combination, the performance and the characteristics such as being a liquid, being a solventless material which does not require a time for drying a solvent in a production process, having low viscosity suitable for application in spite of being a solventless material, and having high brightness, that is, a high refractive index for a resin, in order to maintain luminosity with a smaller amount of light.
A technology to produce an optical sheet by using such a material that has a high refractive index and includes, for example, urethane acrylate, which is obtained by reacting bisphenol F diethoxy glycol, tolylene diisocyanate, and 2-hydroxyethyl acrylate and which has high aromaticity, as a primary component in the molecular structure is known (for example, refer to PTL 1).
The optical sheet described in PTL 1 has high aromaticity of the resin material, and a refractive index of about 1.55 can be realized. However, under the present circumstances, a high refractivity, which is particularly required in recent years and which is required for increasing the brightness of the lens sheet, is not satisfied. Consequently, in the case where an isocyanate compound, for example, diphenylmethane diisocyanate, having higher aromaticity is used for the purpose of realizing a higher refractive index, the resin material itself becomes highly viscous or is crystallized, and forming into an optical member becomes difficult.
Meanwhile, it is proposed to produce a low-viscosity high-refractive-index cured product by using a urethane (meth)acrylate having an open circular structure of lactone, a bisphenol structure, and at least two radical-polymerizable unsaturated groups in the molecule (for example, refer to PTL 2).
The bisphenol structures described in PTL 2 are bisphenol A, bisphenol F, and bisphenol S and the refractive indices thereof are still about 1.55. In the case where a sulfur atom is further included, a urethane (meth)acrylate produced by using this may be colored in itself, or a light resistance problem may occur when a lens is formed. Therefore, application to such use is difficult.